Color LCD with microcompensators and methods for their formation

ABSTRACT

A method for forming an assay of microcompensators having a pattern of sub-pixel-sized regions of a light sensitive resin, which has been dispersed a colored pigment, aligned with the microcompensators for phase compensation.

This is a division of patent application Ser. No. 09/262,304, filingdate Mar. 4, 1999 now U.S. Pat. No. 6,160,599 issued Dec. 12, 2000 whichis division of patent application Ser. No. 08/742,102 now U.S. Pat. No.5,929,955. filed Oct. 31, 1996 and issued Jul. 27, 1999, Color LCD WithMicrocompensators, assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to the general field of Liquid Crystal Displays,particularly color balance in the emergent light.

(2) Description of the Prior Art

Referring to FIG. 1, the basic parts of a liquid crystal display areschematically illustrated in cross-section. A number of layers areinvolved, the outermost being a pair of crossed polarizers 1 and 2. Intheir most commonly used configuration, the polarizers are arranged soas to have their optic axes orthogonal to one another. That is, in theabsence of anything else between them, light passing through theentrance polarizer would be blocked by the exit polarizer, and viceversa.

Below the entrance polarizer 1 is an upper transparent insulatingsubstrate 3 (usually glass) and immediately above the exit polarizer 2is a similar lower substrate 4 . Conducting lines, such as 6, runningorthogonal to, and insulated from, one another are located on the upper(inward-looking) surface of 4. Said orthogonal lines are connected attheir intersections, optionally through Thin Film Transistors (TFTs).This allows voltage, separately applied to a pair of orthogonal lines,to be added together only at the intersecting position which willoverlie a given pixel (or sub-pixel) of the display.

Sandwiched between, and confined there by means of suitable enclosingwalls (not shown), is a layer of liquid crystal 5. Liquid crystalscomprise long thread-like molecules whose orientation, relative to agiven surface can be controlled by coating such a surface with asuitable orientation layer (not shown) and rubbing said orientationlayer in the desired direction just prior to bringing it into contactwith the liquid crystals.

Thus, in FIG. 1, the molecules closest to upper substrate 3 might beoriented so as to lie in the plane of the figure while the moleculesclosest to lower substrate 4 would be oriented to lie perpendicular tothis plane. Molecules in between the two sets of pre-oriented moleculesthen arrange themselves so as to gradually change their orientationbetween these two extremes. Hence the term ‘twisted nematic’ (TN) forsuch a configuration. A TN is optically active and will rotate the planeof any polarized light that traverses it.

Thus, polarized light that was formed and oriented as a result ofpassing through entrance polarizer 1 will be rotated though an angle(for example 90°) after traversing layer 5 and so will be correctlyoriented to pass through the exit polarizer 2. Such a device istherefore normally on (transmits light).

An important property of TN is that, in the presence of an electricfield (typically about 1,000 volts/cm.), normal to the molecules, saidmolecules will all orient themselves so as to point in the samedirection and the liquid crystal layer will cease to rotate the plane ofpolarization. As discussed above, a single pair of orthogonal linescomprise one electrode for generating said electric field, the otherbeing transparent conducting common electrode 10, comprisingindium-tin-oxide (ITO).

Besides the TN structure, there is another possible configurationwherein all molecules of liquid crystal 5 (in FIG. 1) are oriented tolie in the plane of the figure but the inclination of each is different.The molecules closest to upper substrate 3 might be inclined ‘right upand left down’ while the molecules closest to lower substrate 4 would beinclined ‘left up and right down’. These inclination angles (7° in ourexample) can be controlled by selecting the orientation layer. Moleculesin the middle position between upper and lower substrates might beperpendicular in the presence of the electric field. Other molecules,between these three sets of molecules, then arrange themselves so as togradually change their orientation (bending) between the threeboundaries. Hence the term ‘bend cell’ for such a configuration.Examples of bend cells are described by C. L. Kuo et al. in Appl. Phys.Letters vol. 68 March 1996 pp. 1461-1463, and by T. Miyashita et al. inJpn. Jour. Appl. Phys. vol. 34 February 1995 pp. L177-L179.

In addition to being capable of rotating the plane of polarization, asin a TN structure, liquid crystals are also birefringent. This meansthat for a plane polarized wave of light there are different refractiveindices for the two components of the electric vector. As a result,after passing through a given thickness of a bi-refringent layer, thephase difference between these components (normally 0) changes,resulting in an elliptically polarized wave.

After passing through the exit polarizer 2, said elliptically polarizedlight is converted once more to plane polarized light. However, itsintensity will have been changed, depending on the value of theafore-mentioned phase change which can be modified by varying thevoltage applied across the liquid crystal molecules.

To view a display of the type illustrated in FIG. 1, light may beapplied from above the entrance polarizer, and viewed from below theexit polarizer or a reflecting surface may be applied to the lowersurface of exit 2 polarizer and the device viewed from above.

In general, color LCDs are built in the same way as monochrome LCDsexcept that their light has been passed through color filters. Thelatter comprise a matrix of sub-pixel size regions, such as 7, 8, or 9in FIG. 1, on common substrate 3, each region being a tiny singlefilter. The spatial locations of the different colored regions are knownto the liquid crystal display control system which determines the amountof light that is allowed to pass beyond any given dot, thereby creatinga color image. For example, in FIG. 1, region 7 might represent a greenfilter, region 9 a blue filter, and region 8 a red filter.

One of the ways in which multicolor filters are manufactured is by usinga light sensitive resin (such as a methacrylate polymer) as the materialout of which the aforementioned dots are formed. Such a resin can bemade to serve as a light filtering medium by dispersing an appropriatepigment within it. Then, by using a mask when exposing such a resin tothe appropriate actinic radiation, any desired pattern ofsub-pixel-sized regions of a given color can be produced.

A commonly incorporated feature of LCDs is a black matrix, (not shown inFIG. 1). Its purpose is to block light that reaches layer 2 withouthaving passed through the open space between color filters. Such lightis extraneous to the display and reduces the overall contrast.

Besides changing the overall brightness of the display, there is a moreserious problem associated with the birefringent phenomenon, asdescribed above. The magnitude of the phase change varies withwavelength so that the relative intensities of the emerging colorsdiffer from that of the original image. This dispersive effect isparticularly noticeable with respect to light that was intended toappear white but, instead, appears slightly colored.

A partial solution to this problem has been described in U.S. Pat. No.5,136,405 (Wada et al. August 1992) and U.S. Pat. No. 5,380,459(Kanemoto et al. January 1995). These patents teach the use of a single(macro) compensator comprising a sheet of bi-refringent material (11 inFIG. 1) that introduces additional phase changes, for the purpose ofcancelling out those introduced by the liquid crystal. Unfortunately,the dispersion of the macrocompensator material cannot be matchedexactly to that of the liquid crystal so exact cancellation of the colordistortion introduced by the liquid crystal is not possible with amacro-compensator and a conventionally designed liquid crystal display.

An attempt to match the dispersion of the liquid crystal layer to thatof a macrocompensator has been described by Haim et al. in U.S. Patent5,402,141 (March 1995). Haim's approach is to modify the dimensions ofthe cell gaps within the display itself so that the phase changes thatare introduced are better suited to correction by a macrocompensator. Aswill be seen, this approach is quite different from that of the presentinvention.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a color LiquidCrystal Display wherein a true color balance is seen for the fullvisible spectrum.

A further object of the present invention has been that said fullspectral range color balance be seen for all viewing angles.

Yet another object of the present invention has been that said fullspectral range color balance be seen for all voltages applied across theliquid crystal layer of the display.

An additional object of the present invention has been to provideseveral different structures, all of which meet the aforementionedobjects.

A still further object of the present invention has been to provide acost effective method for manufacturing the afore-mentioned structures.

These objects have been achieved by providing a separate set ofmicrocompensators for each primary color that comprises the display. Bythis means, an exact match between the color distortion introduced bythe liquid crystal layer and the phase compensation needed to correctthis can be made for each primary color separately. Several differentarrangements of the micro color filters of the display and theircorresponding micro-compensators are shown and a cost effective methodfor manufacturing these structures is described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is cross-section through a color Liquid Crystal Display of theprior art, including a single macrocompensator for improving colorbalance.

FIGS. 2 through 5 show different embodiments of the present invention,featuring individual microcompensators for the different colors.

FIG. 6 is a plan view of FIG. 5.

FIG. 7 is a series of curves showing transmittance of green and bluelight vs. viewing angle for a display of the prior art.

FIG. 8 is a series of curves showing transmittance of green and bluelight vs. viewing angle for a display based on the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As already discussed, birefringent materials such as liquid crystalshave different refractive indices for the two components of aplane-polarized light wave. Furthermore, said refractive indices varywith wavelength in a manner that differs from one material to the nextso that exact compensation for the dispersive effects of the liquidcrystal is not possible with a compensator that comprises a singlematerial and thickness.

The approach taken by the present invention is to provide each sub-pixel(and hence each color of the display) with its own individualmicrocompensator. The composition and thickness of each microcompensatorcan then be tailored to provide exactly the degree of compensationneeded for that color alone.

Referring now to FIG. 2, we illustrate there a first embodiment of thepresent invention. Those portions of the figure that are not novelcontinue to be designated with the same numbers as were used in FIG. 1.Microcompensators 17, 18, and 19, designed to compensate for phasechanges in red, green, and blue light, respectively, have been depositedon the lower (inward-looking) surface of substrate 3. Saidmicrocompensators comprise one or more materials taken from the groupconsisting of polycarbonates, polyethylene-terphthalate, polyvinylalcohol, polysulphones, and polyimides, and will have thicknesses thatdiffer one from another, but are in the range of from about 1 micron toabout 100 microns.

The first set of microcompensator's (for example 19) is formed by theapplication of birefringent material to the surface of 3, by means ofspin-coating (at between about 500 and 2,000 RPM), to the desiredthickness level, followed by baking at a temperature between about 25and 120 ° C. for between about 5 and 10 minutes in an atmosphere of air.A light sensitive resin (such as meth-acrylate polymer), within whichhas been dispersed a colored pigment (chosen to match the phase changeof the micro-compensator on which it has been deposited), is thenapplied, also by spin-coating, to a thickness of between about 0.5 and10 microns. Said resin layer is now exposed, through a suitable mask,and developed, leaving behind sub-pixel-sized areas of resin 27, 28, and29.

An etchant which does not attack the developed resin is now used toremove all birefringent material not covered by resin. The assemblage isthen given a second heat treatment at a temperature between about 25 and120° C. for between about 5 and 10 minutes in an atmosphere of air.Subsequent sets of microcompensator/color resin pairs are then formed byrepeating the above steps with appropriate changes in material thicknessand/or composition. Following the above steps, the structure has theappearance shown in FIG. 2.

Note the presence of macrocompensator 11 in FIG. 2. The incorporation ofa macrocompensator, in adition to the microcompensators 17, 18, and 19is not essential for the effective operation of the present inventionbut is optional. If used, its purpose would be to reduce the degree ofphase change that would need to be provided by the microcompensators.

A second embodiment of the present invention is shown in schematiccross-section in FIG. 3. For simplification purposes, a number ofstandard components that form part of the complete Liquid CrystalDisplay, that were shown in FIGS. 1 and 2, are no longer shown here.These include the crossed polarizers, the field generating layers, andthe optional macrocompensator. FIG. 2 illustrates that themicrocompensators do not have to be in physical contact with the colorfilters and may even be located on different substrates.

In FIG. 3, microcompensators 37, 38, and 39 reside on the upper(outward-looking) surface of substrate 3 while color filters 137, 138,and 139 reside on the upper (inward-looking) surface of substrate 4. Allthat is required is that emerging light that passes through the colorfilters also passes through the microcompensators. This structure offersthe advantage that microcompensators and color filters may be preparedin parallel, rather than serial, processes and the disadvantage that aseparate photoresist step will be needed for the formation of themicrocompensators. Additionally, the upper and lower substrates willneed to be carefully aligned during assembly of the full display.

FIG. 4 illustrates a third embodiment of the present invention whereintwo of the colors (passing through filters 137 and 138) share a commonmicrocompensator 47 while light passing through filter 139 continues tohave its own micro-compensator 49. In practice, the preferredapplication of this embodiment would be for red and green light to sharea micro-compensator, with blue light having its own. The advantage ofthis structure is that fewer manufacturing steps would be needed.

A fourth embodiment is shown in FIG. 5. Each color has its ownmicrocompensator but each of the latter now comprises two adjacentregions. Said regions may be of different thickness, different material,or both. Because the size of a micro-compensator is well within theresolution of the human eye, the light transmittance provided by eachregion separately is averaged out by the eye, in proportion to therelative areas of the two regions. Thus, through correct choice of therelative areas of the two regions, a multiplicity of differentmicro-compensators may be formed while using only a two-step process.FIG. 6 is a plan view in which cross-section AA comprises FIG. 5.

The effectiveness of the present invention is illustrated by thefollowing example:

FIG. 7 represents typical transmittance vs. polar viewing angle data forblue and green light in which a single (macro) compensator, associatedwith a bend cell, was used. Curves 71 through 75 are for green light inwhich the voltages applied across the liquid crystal layer were 5.98,2.84, 2.30, 1.90, and 1.42 volts respectively, while broken curves 171through 175 are for blue light in which the voltages were 5.98, 3.23,2.68, 2.25, and 1.66 volts respectively.

FIG. 8 represents transmittance vs. polar viewing angle data which weobtained by simulation for blue and green light in which each color hadits own microcompensator. Curves 81 through 85 are for green light inwhich the voltages applied across the liquid crystal layer were 5.98,2.84, 2.30, 1.90, and 1.42 volts respectively, while broken curves 184and 185 are for blue light (2.25 and 1.66 volts respectively). Curvesfor blue light for the three lower voltages were too close to the greenlight curves to be distinguished in the figure. As can be seen, theimprovement associated with the use of microcompensators in place of amacrocompensator are considerable.

While the invention has been particularly shown and described withreference to these preferred embodiments, it will be understood by thoseskilled in the art that various changes in form and details may be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for forming an array ofmicrocompensators, comprising: (a) providing a substrate; (b) depositinga birefringent layer on said substrate; (c) heating said birefringentlayer and said substrate; (d) depositing a layer of a light sensitiveresin within which has been dispersed a colored pigment; (e) exposingand then developing said light sensitive resin to form a pattern ofsub-pixel-sized regions and then, using an etchant which does not attackthe developed resin, removing all birefringent material not covered bythe resin; (f) heating said birefringent layer, said developed resin,and said substrate; and (g) optionally repeating steps (b) through (f)at least one more time.
 2. The method of claim 1 wherein eachbirefringent layer comprises material taken from the group consisting ofpolycarbonates, polyethylene-terphthalate, polyvinyl alcohol,polysulfones, and polyimides.
 3. The method of claim 1 wherein at leastone of said birefringent layers is deposited by means of spin coating.4. The method of claim 1 wherein the thickness of each birefringentlayer is between about 1 and about 100 microns.
 5. The method of claim 1wherein step (c) and step (f) further comprise heating at a temperaturebetween about 25 and 120° C. for between about 5 and 10 minutes in air.